UPDATE // JANUARY 31:
The contents of a text message Kesha sent to Lady Gaga last February about Dr. Luke have been revealed. According to newly-filed legal documents obtained by Buzzfeed, the text “falsely stated that another female recording artist had been raped by Gottwald, and again repeated the baseless assertion that Kesha had been raped by Gottwald.” Luke adamantly denies the claim and is asking a judge to allow him to add a defamation claim to his ongoing lawsuit against Kesha for the “malicious purpose of further damaging [Dr. Luke’s] reputation and business.”
The text that says the other recording artist was “raped by the same man” did not specifically state his name, but Luke's legal team says it's clear he's the one being referred to.
Previous reports (see below) attached Katy Perry's name to the lawsuit, so perhaps the unnamed recording artist Kesha is referring to is her, though that is not verified or confirmed. Perry's name is not mentioned anywhere in Buzzfeed's new report. It was claimed a judge granted Dr. Luke the permission to show Katy Perry the text in question.
Kesha's legal team also filed papers on Monday – they assert Luke has a “vendetta against Kesha,” is blocking her from releasing new music and is withholding owed payments.
From Buzzfeed:

Over the past 10 months “Kesha’s efforts to record and release her next album have been met with unfounded resistance and delay,” according to her court papers. While she handed over 20 songs this summer, not one song has been approved, no release date has been set for the album, and no promotional plan has been finalized for a release, her attorneys argued.
Per the exclusive recording contract she signed in 2005 when she was 18, Kesha must deliver five albums for Dr. Luke before their arrangement is terminated. Until she delivers three more, she could be locked into the contract for life. Kesha’s last album, Warrior, was released Nov. 30, 2012.
While Kesha no longer has to work in the same room as him, her attorneys said she is still under his control. The “untenable” situation will only get worse when Dr. Luke’s contract with Sony ends in March leaving her completely under Dr. Luke’s management control, bound to him until she completes three more albums. Kesha, they said, cannot be expected to work for a man “who has verbally abused her, made physical threats against her, and sued her and her mother for punitive damages in an attempt to bankrupt them both.”
“You can get a divorce from an abusive spouse. You can dissolve a partnership if the relationship becomes irreconcilable. The same opportunity — to be liberated from the physical, emotional, and financial bondage of a destructive relationship — should be available to a recording artist,” Kesha’s court papers state.
Given their history, Kesha’s attorneys said it is “impossible” for her and Dr. Luke to work together, and asked the court “in the interest of justice” to free her from the contract.

EARLIER // JANUARY 4:

Lady Gaga and Katy Perry have been dragged into Kesha's sexual assault suit against Dr. Luke.

Kesha's New Year's Resolution is to use her phone/Internet less, and for good reason; she'll avoid reports like this, which detail bizarre twists and turns in her legal spat against producer Dr. Luke.
A private text message from Kesha to Gaga sent in April 2016 was added as evidence in Kesha's ongoing sexual assault case against Dr. Luke. Curiously, a judge is granting Dr. Luke permission to show the text to Katy Perry, though it's unknown why or how Perry is related; the contents of the text are sealed, but it does imply that Perry was mentioned in an exchange between the two pop stars.
The Daily Mail reports: "The evidence was presented as part of a hearing on December 26 in New York, when both parties worked to hash out an agreement on discovery, deciding which documents and records they will exchange with one another."
Gaga has been an outspoken advocate for Kesha over the year. "I feel like she's being very publicly shamed for something that happens in the music industry all the time, to women and men," Gaga previously said. "I just want to stand by her side because I can't watch another woman that went through what I've been through suffer."
Perry, who collaborated with Dr. Luke throughout most of her career, has yet to offer any insight, but as they say... silence is deafening.